Tech Talk #29 – Security Clearance

By David Reher, Reher-Morrison Racing Engines

“Almost without exception, engine problems are the result of insufficient clearances.”

We are often told to think big if we want to achieve success, but today I want you to think small. In fact, I want you to think about clearances in a racing engine that are smaller than one thousandth of an inch. These minuscule measurements can mean the difference between long life and sudden death for a racing engine.

Every year we see hundreds of motors come through Reher-Morrison Racing Engines. Some are built by professionals, others are assembled by racers in home workshops. Almost without exception, the problems that we find are the result of insufficient clearances. I’ve come to the conclusion that a little extra clearance is infinitely better than not enough clearance.

There are hundreds of parts in a racing engine. Most of them rub, slide or rotate against another component. In a perfect world, there is no metal-to-metal contact between parts; instead, chains of oil molecules separate the two like atomic roller bearings. (The petroleum experts may disagree with my analogy, but that’s my layman’s explanation.)

A hydrodynamic wedge of oil can withstand thousands of pounds of pressure while producing very little friction. But if this lubricating film breaks down due to inadequate clearance, friction and pressure create heat that microwelds the two surfaces together. As the parts continue to move against each other, pieces of metal are ripped from their surfaces, further accelerating the destructive process. Eventually the two parts seize, producing the blackened and burned appearance that we see in spun bearings, scuffed pistons and galled lifter bodies.

So how can engine builders keep the hundreds of parts in a motor from reaching this dire condition? By providing enough clearance for the lubricating molecules to do their job.

The cardinal rule of engine building is to measure every clearance. The one dimension that you assume is correct is the one that will destroy your engine. At our shop we use build sheets to record every vital dimension – main bearing bore diameters, bearing thicknesses, crankshaft journal diameters, connecting rod bores, and on and on. Whether you build one engine or 50 engines in a season, filling out a build sheet reminds you to check every piece that should be checked. If you don’t want to design your own assembly records, the Chevrolet Power manual has sample sheets that you can photocopy and use for your engine building projects.

I encourage you to purchase a good set of micrometers, a dial indicator and calipers. The price of precision tools has dropped dramatically, and good tools are an investment that lasts a lifetime. If you prevent just one catastrophic failure by finding an out-of-tolerance clearance, you’ve paid for the cost of your tools.

All manufactured components are built to tolerances. By checking parts before assembly, we learn exactly what those tolerances are. For example, your machinist may finish your block’s main bearing housings on the small side of the specified plus-or-minus dimension. The crankshaft you bought from a friend may be on the high side of the range of outside diameters. Both parts are within their respective tolerances, but in combination they can produce bearing clearances that are dangerously tight. It’s better to find a problem like this on the workbench than to find it when pieces of bearing are lying in the bottom of the oil pan.

One of the persistent problems we encounter is piston ring end gaps that are too small. Many racers are squeezing the end gaps in order to improve leakdown test results. Well, if you want to see zero leakage on your meter, just put O-rings on your pistons. But if you want to make power where it counts – on the race track – you must have enough clearance to prevent the ends of the piston rings from butting under actual operating conditions. Even if you don’t reach the point of damaging the cylinder walls, you risk impairing the ring seal if the end gaps are too small. In our Pro Stock engines, we run generous .028-inch gaps on the top rings. Believe me, if I thought there was power in tighter end gaps, I’d reduce the clearance in a heartbeat.

Wrist pin galling is becoming an epidemic in racing engines, and I believe the causes are insufficient pin clearance and inadequate lubrication. Today’s dry-sump oil pans are very efficient at scavenging oil, and vacuum pumps further reduce the amount of oil circulating in the crankcase. The unintended consequence of these improvements is that the wrist pins, which depend entirely on splash lubrication, are now starved for oil. We mill slots in the cheeks of the connecting rods in our Pro Stock engines to direct a spray of oil to the underside of the pistons; this both cools the pistons and lubricates the pins.

Valve guide clearances that are too tight are another common problem. I think it’s a mistake to reduce the valve guide clearance in the hope that you can control where the valve seals on its seat. In fact, the valves are bending, flexing and bouncing like crazy at high rpm. I’ve never seen a valve stick in its guide with .002-inch clearance, but I’ve seen lots of them stick with tight clearances. My advice is to play it safe and open up the clearance; you might even find a little power.

It’s become fashionable to brag about tight piston-to-wall clearance because this supposedly stabilizes the pistons and improves power. But where are you measuring the clearance? One piston manufacturer may specify the clearance under the oil ring, while another might specify it at the skirt tang. With tapered and barrel-ground skirts, where you measure the piston diameter makes a huge difference.

If you see heavy outlines of the piston skirts on the cylinder walls, that is a clear warning sign that the piston-to-wall clearance is too tight. From the first day that I installed a set of custom pistons, I’ve found that a little extra clearance never hurts. More clearance definitely adds a safety margin in case you misinterpret the manufacturer’s measurement instructions – and in most instances, it increases power, too.

You do check your lifter clearances in the block, don’t you? Aggressive cam profiles and steeply angled pushrods impose high side loads on the lifters. You can help keep them alive by making sure that they have adequate clearance in their bores. Repairing a galled lifter bore isn’t cheap, and the damage done by a broken lifter is never pretty. Please do me a favor and check your lifter clearances.

Building an engine is like painting a car: the real work is in the preparation. A drag racing engine only runs in competition for a few seconds at a time, but it can take days to check and assemble it properly. Whether you are a professional or a novice, it’s time well spent.